Solenoid

How does a solenoid work?

A
solenoid is simply a specially designed electromagnet. A
solenoid usually consists of a coil and a movable iron core called
the armature. Here's how it works.When current flows through a
wire, a magnetic field is set up around the wire.If we make a coil of
many turns of wire, this magnetic field becomes many times stronger,
flowing around the coil and through its center in a doughnut
shape.When the coil of the solenoid is energized with current, the
core moves to increase the flux linkage by closing the air gap
between the cores. The movable core is usally spring-loaded to allow
the core to retract when the current is switched off. The force
generated is approximately proportional to the square of the current
and inversely proportional to the square of the length of the air
gap.

Solenoids are inexpensive, and their use is primarily limited to
on-off applications such as latching, locking, and triggering. They
are frequently used in home appliances (e.g. washing machine valves),
office equipment (e.g. copy machines), automobiles (e.g. door latches
and the starter solenoid), pinball mahines (e.g., plungers and
bumpers), and factory automation.

Applications

An
electromechanical relay is a solenoid used to make or break
mechanical contact between electrical leads. A small voltage input to
the solenoid controls a potentially large current through the relay
contacts. Applications include power switches and electromechanical
control elements. A relay performs a function similar to a power
transistor but has the capability to switch extremely large currents
if necessary. However, transistors have a much shorter switching time
than relays.

As illustrated in figure 2, a voice coil consists of a coil
that moves in a magnetic field produced by a permanent magnet and
intensified by an iron core. The force on the coil is directly
proportional to the current in the coil. The coil is usually attached
to a movable load such as the diaphragm of an audio speaker, the
spool of a hydraulic proportional valve, or the read-write head of a
computer disk drive. The linear response and bidirectional capability
make voice coils more attractive than solenouds for control
applications.